The present invention relates to methods and arrangements in a mobile communication network adapted to use re-transmissions of the type Hybrid Automatic repeat request (HARQ). An example of such a communication network is a UMTS terrestrial radio access network (UTRAN). The UTRAN is illustrated in FIG. 1 and comprises at least one Radio Network System 100 connected to the Core Network (CN) 200. The CN is connectable to other networks such as the Internet, other mobile networks e.g. GSM systems and fixed telephony networks. The RNS 100 comprises at least one Radio Network Controller 110. Furthermore, the respective RNC 110 controls a plurality of Node-Bs 120,130 that are connected to the RNC by means of the Iub interface 140. Each Node B covers one or more cells and is arranged to serve the User Equipment (UE) 300 within said cell. Finally, the UE 300, also referred to as mobile terminal, is connected to one or more Node Bs over the Wideband Code Division Multiple Access (WCDMA) based radio interface 150.
Requirements for mobile data access are increasing and demand for bandwidth is growing. To meet these needs the High Speed Data Packet Access (HSDPA) specification has been defined. HSDPA is based on WCDMA evolution standardized as part of 3GPP Release 5 WCDMA specifications. HSDPA is a packet-based data service in WCDMA downlink with data transmission peak rate up to 14.4 Mbps over a 5 MHz bandwidth. Thus HSDPA improves system capacity and increases user data rates in the downlink direction. The improved performance is based on adaptive modulation and coding, a fast scheduling function and fast retransmissions with soft combining and incremental redundancy. HSDPA utilizes a transport channel named the High Speed Downlink Shared Channel (HS-DSCH) that makes efficient use of valuable radio frequency resources and takes bursty packet data into account. This is a shared transport channel which means that resources, such as channelization codes, transmission power and infra structure hardware, is shared between several users. HS-DSCH supports HARQ as a fast and resource-efficient method for combating transmission errors.
In 3GPP Release 6, the WCDMA standard is further extended with the Enhanced Uplink concept by introducing the Enhanced Dedicated Transport Channel, E-DCH. A further description can be found in 3GPP TS 25.309 “FDD Enhanced Uplink; Overall description”. This concept introduces considerably higher peak data-rates in the WCDMA uplink. Features introduced with E-DCH include fast scheduling and fast Hybrid Automatic Repeat reQuest (HARQ) with soft combining. Fast scheduling means that the Node B can indicate to each UE the rate the UE is allowed to transmit with. This can be done every TTI, i.e. fast. Thus, the network is able to control the interference in the system very well.
HARQ is a more advanced form of an ARQ retransmission scheme. In conventional ARQ schemes the receiver checks if a packet is received correctly. If it is not received correctly, the erroneous packet is discarded and a retransmission is requested. With HARQ the erroneous packet is not discarded. Instead the packet is kept and soft combined with the retransmission. That implies that even if neither the first transmission nor the retransmission would facilitate a successful decoding when received alone, they may be combined to decode the packet correctly. This means that, compared to conventional ARQ, less transmission power and fewer retransmissions are required on average.
Voice over IP (VoIP) and support for Internet Multimedia Subsystem (IMS) over the new channels like HS-DSCH and E-DCH poses particular challenges. This is because the efficiency of a realization, and the capacity limit, is stressed by a large number of users, all of which are only injecting a low traffic volume with a lot of small packets. Thus, to optimize cell/network capacity, it is therefore very important to minimize the overhead of each user and each packet. This overhead includes both protocol overhead and signalling overhead. The present invention concerns the signalling overhead issue. The reliability of each link could potentially be slightly sacrificed in the quest for reducing overhead.
One of the main differences of the uplink DCH according to Release 99 uplink and E-DCH according to Release 6 is the fact that E-DCH supports HARQ. This means that the “average” transmission power for E-DCH can be kept lower, because stochastic transmission errors due to fading are corrected by the fast HARQ which is located in the Node B. HARQ also uses soft-combining resulting in a high power-efficiency with early-termination gain, etc. These aspects make the E-DCH an attractive and efficient solution.
However, HARQ also implies a cost. Compared to DCH, both the protocol- and the signalling overheads are larger for E-DCH. Thus, in order to make a VoIP-over-E-DCH realization a competitive alternative, it is important to make sure that these costs do not exceed the gains.
This invention relates to an improved HARQ realization and particularly an improved HARQ realization for the E-DCH uplink in WCDMA network and for the downlink channel in HSDPA, HS-DSCH. It should be noted that the invention concerns any system with HARQ.
The state-of-the-art ACK/NACK feedback mechanism is now briefly described. The HARQ is based on multiple interleaved synchronized stop-and-wait ARQ processes with soft combining as illustrated in FIGS. 2-4. The receiver, e.g. the Node B in the case of enhanced uplink, responds to a transmission with an ACK or NACK, so that a successful decoding results in ACK feedback, and unsuccessful decoding results in NACK feedback. To a NACK response, the sender, e.g. the UE is “re-transmitting” on the same HARQ process by providing additional power/redundancy to the decoding process in the receiver. The receiver performs soft combining of the multiple HARQ transmissions. An ACK reception in the UE results in a termination of the HARQ in that process, and that process can then be utilized for transmitting new data. The HARQ feedback is in the case of enhanced uplink carried over an E-DCH HARQ Acknowledgement Indicator Channel (E-HICH). E-DCH supports Macro Diversity, also referred to as soft handover, i.e. there can be multiple cells receiving data and sending feedback to the UE.
HARQ is also used for HS-DS H. HARQ for HS-DSCH is similar to HARQ for E-DCH. It should however be noted that HS-DSCH does not support soft handover and fast power control.
FIG. 3 illustrates the HARQ behaviour for a single process. Here, most transmissions are successful with a single transmission, except block #3, which require two transmissions. The subscript denotes the retransmission sequence number.
FIG. 4 illustrates the HARQ behaviour when the decoding requires several HARQ transmissions. The first block is successfully decoded after three HARQ transmissions, while the second block required only two and the third block needed four transmissions for successful decoding.
A challenge in the operation of HARQ is to achieve sufficient reliability of the HARQ feedback without spending a lot of resources on the ACK/NACK feedback. For small transport blocks in particular, the relative overhead of this signalling may be quite costly.